Unrestricted Hartree-Fock 4-31G calculations were performed on carbon-centred radicals to analyze the influence of substituents on the static and dynamic properties of these systems. dd-substituted radicals are not planar and little stabilized. cc (or cd)-substitution leads to planar systems which are highly stabilized. The formation of sigma -radicals by hydrogen abstraction from the parent molecule is characterized by a smaller exothermicity and presumably a higher activation energy than the formation of pi -systems. The autoreaction of alkyl radicals substituted by two captor groups is thermodynamically less favoured than their addition to olefins. Capto-dative substitution seems to favour the whole process: 2RH rarr 2R rarr R-R.